The construction of reed valves for hermetic compressors directly affects their energy and volumetric efficiency. One of the losses is the energy loss due to overpressure related to the opening readiness of the discharge valve, after the discharge pressure has been reached within the compression chamber of the cylinder. In situations where the discharge valve does not open promptly, a condition of overpressure will occur inside the cylinder compression chamber. The longer the part of the compression cycle during said overpressure condition, the higher will be the effort and the power loss that the compressor crankshaft will have to overcome.
Another loss is the energy loss related to the opening readiness of the suction valve when the pressure inside the cylinder suction chamber reaches the suction pressure. If the suction valve does not open promptly, an underpressure condition will occur inside the cylinder suction chamber and the suction process will be delayed, causing energy and gas volume losses in the compressor.
There is also loss due to back flow, i.e. a mass loss relative to the closing readiness of the valve in the processes or operational steps of suction and discharge in the compressor.
From the above-mentioned, it can be said that a careful definition of the construction characteristics of the reed valves is one of the most important aspects in the hermetic compressor design. Any reed valve will present certain characteristic equations that control its behavior (motion). By simplifying the analysis using a model of mass and spring, we can say that the reed valve motion is defined by the following equations: ##EQU1## EQU .epsilon.=C/(2mfn)=valve damping factor (3),
where:
x=valve displacement PA1 F=pressure force on the valve PA1 m=valve mass PA1 k=valve stiffness PA1 t=time PA1 high fn; PA1 low k; PA1 low inertia (small mass); and PA1 damping effect (specific for each design) PA1 .rho.=specific weight PA1 E=modulus of elasticity
Through the definition of fn and .epsilon., we may conclude that the valve response is in some measure determined by its geometry (dimensions) and material properties.
As mentioned above, the proper response of the valves in a compressor will strongly affect its performance. Considering an ideal operation, it can be concluded that the motion response of a reed valve to an optimum performance of the compressor will be attained by:
a total and immediate opening of the valve, as soon as the suction and discharge pressures are reached; PA0 a situation in which the valve, after opening, is maintained fully opened till the flow is over; and PA0 avoiding the high range fluctuations of the valve and the instability of the valve motion.
Furthermore, the valve must close rapidly after the suction or discharge process has ended in order to avoid back flow losses and, consequently, a decrease in the volumetric efficiency of the compressor.
This theoretical ideal performance is efficient in terms of power consumption, due to the reduction in the head loss of the gas flow through the gas discharge passage and the valve, and also in terms of an increase in the mass of volumetric efficiency, since it avoids any back flow loss and diminishes the delay in the opening of the valve.
A valve with the properties listed below can approximate the ideal motion conditions above mentioned.
All these properties are strongly dependent upon the material of the valve.
The high fn (natural frequency) is desired for a quick response of the valve to avoid the back flow losses. The small valve stiffness is desired to reduce overpressure (discharge) or underpressure (suction) which are necessary to open the valve and which result in energy loss in both cases, and also mass loss in case of suction. The small mass (specific weight) is necessary to reduce the valve inertia, so that the valve can respond more properly to pressure force, avoiding high amplitude fluctuations.
It should be noted that the general behavior of a valve is a function of k and fn, which are determined by the relationship between the modulus of elasticity, specific weight and material strength, as shown below. ##EQU2## where: .sigma. adm=admissible tension of the material
Thus, in order to obtain a valve with a high natural frequency (fn) together with a reduced stiffness (k), the only possibility is to use a material that simultaneously presents a low specific weight, low modulus of elasticity and high strength.
Conventional reed valves for a hermetic compressor are usually made of hardened and tempered steel. One problem of the steel reed valves is that, for specified values of .sigma. adm and E, they present a high specific weight, thereby not being possible to minimize the k/fn relation. Thus, in order to obtain a desirable relation of high natural frequency and low stiffness for these known steel reed valves, the valve thickness has to be reduced, negatively affecting the valve strength. These limitations of the steel reed valves of the prior art deviate the valve performance from the ideal standard of response in terms of motion and cause a certain efficiency loss in terms of energy and mass of the compressor, as explained above.